ATMs are used, for example, to dispense cash and for depositing cash or cheques. Typically, a customer operates an ATM by inserting a magnetic stripe card, encoded with the customer's personal and account data, into a slot in the fascia of the machine, behind which is a card reader.
Fraudsters have devised a number of schemes for stealing customers' magnetic card data. One scheme, known as “skimming”, involves fixing a magnetic read head to the fascia of a machine, which reads the magnetic stripe on a customer's card as it is inserted into or retrieved from the machine, and stores and/or relays the read data to a remote location.
Anything applied, fixed, fitted or adhered to or that settles on, engages or contacts the fascia of an ATM after it has been commissioned, that was not put there by an authorised party for a legitimate reason, is called a “foreign body”, although the term “body” includes not only solid objects but also liquids in whatever form (droplets, films, puddles etc.). There are two types of foreign bodies: those that are non-benign, that is, intended to defraud, cause disruption or deception, such as magnetic read heads, and those that are benign, such as rainwater.
Principally with a view to counteracting fraud, ATMs have been equipped with sensors that detect foreign bodies. One particular sensor works by determining changes in capacitance caused by a foreign body. However, both non-benign and benign foreign bodies can cause capacitance changes. Ideally, therefore, it is important to be able to distinguish between them.
Early magnetic read heads tended to be relatively big, which meant they would cause a significant capacitance change. Consequently, they could be distinguished from, for example, rainwater, which does not have such a dramatic effect on capacitance, by setting a high threshold. But as read head technology has developed and heads have become a lot smaller, it has been necessary to lower the detection threshold to the extent that it is now difficult to distinguish between read heads and, for example, rainwater.